The invention relates to controlling vibration of rotary knife cylinders that are used to cut individual sheets in succession from a longitudinally traveling continuous web of material, such as corrugated paperboard. In particular, the invention involves the use of electronically controlled electromagnets to actively damp vibrations, particularly at the natural resonance frequency of the respective rotary knife cylinders.
The invention arose during continuing developmental efforts by the assignee of the present application in seeking improved vibration control for rotary machinery, namely rotary knives as disclosed in copending patent application Ser. No. 09/045,466, now U.S. Pat. No. 6,032,558, entitled xe2x80x9cA Rotary Knife With Active Vibration Controlxe2x80x9d, by James R. Michler, incorporated by reference herein.
In the corrugated paperboard industry, long machines comprised of several components are used to make individual pieces of corrugated paperboard from rolls of craft paper and adhesive. In these machines, a rotary knife is used to cut a continuous web of corrugated paperboard into individual sheets towards the end of the manufacturing process. These individual sheets of corrugated paperboard are then normally stacked and transported for finishing. The quality of the individual sheets and in turn the quality of the end product made from the corrugated sheets depends in large part on the performance of the rotary knife.
In a typical rotary knife, a pair of rotating knife cylinders extend generally transverse across the web. The outer cylindrical wall of each cylinder includes a cutting blade that is helically mounted to the respective cylinder. Gears for the cylinders ensure that the cylinders rotate symmetrically so that the blades engage the web and each other to cut the web transversely in a scissors-like fashion. In order to provide a high quality cut, it is important that the blades tightly engage one another when making the scissors cut. A control system for the knife precisely controls the speed of the rotating cylinders so that the forward velocity of the blade during the cut matches the forward velocity of the web through the knife. In addition, the desired length of the individual sheets for the particular order is regulated by controlling the speed of the rotating cylinders when the blades are not engaged, which is in general a different speed than when the blades are engaged to make the cut. This type of variable speed operation is conventional in the art.
Due to the above-described variable speed operation, low inertia rotary knives are used by the assignee of the present application primarily to improve motor drive control and durability at high speeds. In these low inertia rotary knife cylinders, an upper stationary support shaft and a lower stationary support shaft are mounted to a frame (normally using retainer mounts). An upper rotatable cylindrical tube is placed concentrically over the upper stationary support shaft and a lower rotatable cylindrical tube is placed concentrically over the lower stationary support shaft. The knife blades are attached to the outer wall of the rotatable cylindrical tubes. Bearings are located between the rotatable cylindrical tubes and the respective stationary support shaft. The rotational inertia of the tubular knife cylinder is significantly less than in systems having solid rotating knife cylinders.
In low inertia rotary knife systems, as well as other rotary knife systems, excessive vibration of the knife cylinders can cause poor quality cuts. Under extreme conditions, the vibrations can sometimes even prevent cuts from occurring altogether. It has been found that cutting frequency has a substantial effect on the amount of steady state vibration, and hence the quality of the cut. When the cutting frequency (number of cuts per second) is an integral, or near integral, multiple of the knife cylinder first natural frequency (number of cycles per second), each successive cut adds to the vibration remaining from the previous cut. The total knife cylinder vibration then builds up to a higher level than at slightly different cutting frequencies. A speed change of less than 1% can change the cylinder vibration level by more than 50%. This being the case, vibration problems are more apparent when the knife is operating at high speeds because there is less time for natural dampening to occur between cutting cycles. In addition, vibration problems are more apparent on webs that require relatively high cutting forces.
In the above-incorporated copending patent application Ser. No. 09/045,466, now U.S. Pat. No. 6,032,558, the assignee of this application discloses an active tuned-mass vibration damping system in order to control vibrations of the knife cylinders. More specifically, that system includes a tuned-mass, and a spring arm having one end coupled to the tuned-mass and the other end coupled to the end of the knife cylinder (e.g. in a low inertia rotary knife, the spring arm is coupled to the end of the stationary support shaft). Actuators, preferably electromagnetic actuators, are mounted to provide force on the tuned-mass in order to actively dampen vibrations of the knife cylinder. Vibrations of the knife cylinder and the tuned-mass are measured using vibration sensors, preferably accelerometers, which transmit signals to an electronic control unit. The electronic control unit calculates command signals, preferably using state space control, to drive the electromagnetic actuators in order to actively dampen vibrations in the associated knife cylinder. It is preferred that the control scheme account for phase shifts in the system due to electronic lags (for example, phase lags in the electromagnetic actuator, processing electronics, etc.) by using adjusted control coefficients in the state space control algorithm. The preferred control scheme is disclosed in copending patent application Ser. No. 09/046,267 filed on Mar. 23, 1998, now U.S. Pat. No. 5,983,168, entitled xe2x80x9cPhase Shift Accommodation for Active Tuned-mass Damping Systemxe2x80x9d, by James R. Michler, assigned to the assignee of the present application, and incorporated herein by references. The command signal is scaled and then output from the electronic control unit to drive the electromagnetic actuators which provide force on the tuned-mass to actively damp vibrations in the knife cylinder.
It is also disclosed in the above-incorporated copending patent application Ser. No. 09/045,466, now U.S. Pat. No. 6,032,558, entitled xe2x80x9cA Rotary Knife With Active Vibration Controlxe2x80x9d, by James R. Michler, that the tuned-mass be preferably made from a plurality of laminated steel plates. The steel plates are aligned in the direction of the magnetic field from the respective electromagnetic actuators. In this manner, any currents transverse to the direction of the magnetic field are not allowed to propagate for substantial distances, and thus are prevented from generating excessive heat in the tuned-mass. In addition, it was preferred to have permanent magnets embedded in the surface of the tuned-mass at locations corresponding to the legs of the respective electromagnetic actuators. In this manner, a single electromagnetic actuator could be used to both push and pull the tuned-mass in the selected direction. The permanent magnets were affixed to the tuned-mass using adhesive.
While the inventions disclosed in the above incorporated copending patent application Ser. No. 09/045,466, now U.S. Pat. No. 6,032,558, and copending patent application Ser. No. 09/046,267 (now U.S. Pat. No. 5,983,168) are effective at reducing rotary knife vibrations, some difficulties have arisen with respect to commercial embodiments of the inventions in certain applications. For example, the commercial systems included eight sensors (i.e., vertical and horizontal accelerometers for both the knife shaft and tuned-mass for both the upper and lower knife cylinders), and occasionally one of the sensors would fail. Upon sensor failure, it was possible for the control system to become unstable and actually increase the total vibration level. If left unchecked, the increased vibration level could damage knife blades. Eliminating the potential for such instabilities is desirable.
In addition to reducing instabilities due to sensor failure, it has also been found that it would be desirable to improve the response time for the system. Time lags in the system using the spring arm and tuned-mass are generally in the range of 90 to 130 milliseconds, whereas the current commercial version of the rotary knife sold by the assignee of this application is capable of making nine cuts per second (i.e. 111 milliseconds per cut). Thus, process lags in the active vibration control system can significantly compromise system performance.
The tuned mass vibration control system has three sources of lags. The first source of lag is a mechanical transmission lag between the knife cylinder and the tuned mass. For example, a change in vibration amplitude and/or phase at the knife cylinder, as caused by making a cut, can take more than 100 milliseconds to propagate through the knife frame to the tuned mass. The vibration control system reduces vibration only after the tuned mass is able to change its vibration amplitude and phase to match the new vibration amplitude and phase of the knife cylinder. The second source of lag is in the observer. The observer takes about 10 milliseconds to respond to a change in vibration amplitude or phase. The third source of lag is the electronic lag through the control system. The control system will typically have about 2 milliseconds of total lag. This lag comes from the accelerometers, from sampling delays, various analog to digital converters, digital filters, and digital to analog converters in the control system, and from the amplifier that is connected to the electromagnets.
In order to improve system performance, it is desirable to reduce the process lags in the active vibration control system, and especially the most significant type of lag which corresponds to the transmission of mechanical energy.
The invention is an active vibration control system for a rotary knife that eliminates the use of the spring arm, the tuned-mass, and the sensors associated with the tuned-mass. The spring arm and the tuned-mass are replaced with a lever arm. The lever arm has one end coupled to the knife cylinder, and a distal end that is suspended freely. Actuators are provided to provide a vibration control force directly on the lever arm in response to command signals from the electronic control unit. The lever arm is sufficiently rigid, preferably a steel rod having at least a two inch diameter (typically three inch to four inch diameter), so that mechanical energy is transmitted essentially immediately from the lever arm to the knife cylinder.
Inasmuch as the lever arm and the knife cylinder are coupled to essentially act as a unitary mechanical component, system response lag is reduced significantly. For example, testing has shown that total system response lag for a vibration caused by a cut on a commercial version of assignee""s rotary knife has decreased from 90 to 130 milliseconds with the tuned-mass and spring arm to approximately 10 milliseconds with the rigid lever arm.
In the preferred system, the distal end of the lever arm is made of a ferromagnetic material, and the actuators are electromagnetic actuators. It is possible, however, to implement the invention with other types of actuators. For example, mechanical, pneumatic, piezoelectric, magnetostrictive or hydraulic actuators which provide vibration control forces directly against the lever arm should be possible at the frequencies necessary to control vibrations in a rotary knife.
In addition, the sensors for the tuned-mass are eliminated, which renders the system more robust (less sensitive) to sensor failure. The preferred active vibration damping system now has four sensors per rotary knife, two for both the upper and the lower knife cylinder with one each in the vertical and horizontal directions (in each axle). If any sensor fails, the total system effectiveness merely drops by about 25%. If all sensors fail, the system becomes totally ineffective, and the knife cylinders respond as if the active vibration damping system had not been installed.
As mentioned, the lever arm is made from a sufficiently rigid material such as steel. Even in the preferred electromagnetic system, no permanent magnets are embedded in the lever arm. Rather, the system is provided with pairs of opposing electromagnetic actuators. Each actuator of the pair provides a pulling force on a ferromagnetic portion of the lever arm in a direction opposed to the other electromagnetic actuator in the pair. Preferably, one pair of electromagnetic actuators is oriented vertically with respect to the lever arm, and another pair is oriented horizontally with respect to the lever arm. The electromagnetic actuators pull on the freely suspended end of the lever arm. It is preferred that the freely suspended end of the lever arm consist of a plurality of laminated steel plates aligned in the direction of the magnetic field from the respective electromagnetic actuators, in order to reduce transverse eddy currents and prevent the generation of excessive heat in the lever arm.
In the prior system using the spring arm and the tuned-mass, approximately 80% of the energy transmitted to the knife cylinder to actively cancel vibrations was provided by the inertia of the tuned-mass, whereas the remaining energy was provided by the force of the electromagnetic actuators. In the present system, which eliminates the spring arm and the tuned-mass, there is virtually no inertial component in the energy transmitted to the knife cylinder for active control. Nevertheless, the freely suspended end of the lever arm moves substantially less than the tuned-mass in the previous system, and this coupled with the removal of the permanent magnets, allows for significantly greater magnetic forces on the lever arm than were possible with the tuned-mass. Therefore, the present system using the lever arm is actually able to provide about the same amount of energy for reducing vibration, but with a much faster response time.
Other advantages and features of the invention should be apparent to those skilled in the art upon inspecting the drawings and the following description thereof.